Successful calvarial re-ossification is a characteristic generally restricted to immature animals and infants younger than 2 years of age. Since this osteogenic capacity rapidly diminishes, older children and adults with non-healing cranial defects due to trauma, craniofacial or neurosurgical operations present a difficult reconstructive challenge and a substantial biomedical burden. Presently, surgeons use a variety of techniques to reconstruct non-healing calvarial defects. While these operations are usually successful, they are complex procedures with numerous limitations (e.g. limited donor tissue availability, donor site morbidity, structural failure and high rates of infection). To address these problems, we are investigating the cellular and molecular mechanisms mediating successful calvarial re-ossification. Our long-term goal is to provide solutions for de novo osseous tissue engineering, replacement and repair. While recent studies have documented an interaction between the dura mater and bone formation during calvarial morphogenesis and cranial suture fusion, the mechanisms underlying immature calvarial re-ossification remain unknown. Based on our previous investigations in murine calvarial morphogenesis and cranial suture fusion, we hypothesize that the immature dura mater acts as an endogenous tissue engineer guiding calvarial defect re-ossification. Drawing parallels between three murine models of calvarial bone induction (i.e. calvarial morphogenesis, posterior frontal suture fusion and calvarial defect healing), we have developed the central hypothesis to be tested in this proposal: immature dura mater supplies the osteogenic cytokines, contains the osteoblastic precursors and interacts with overlying bone to successfully engineer calvarial re-ossification. We will test this hypothesis by determining the differences between immature and mature dura mater as well as young and old osteoblasts. Furthermore, we will investigate dura mater-osteoblast paracrine signaling and the effects of equibiaxial strain on dura mater and, in turn, osteoblast biology and gene expression. Finally, we will determine the origin of the osteoblasts that contribute to calvarial re-ossification.